mysql - PHP PDO query returns inaccurate value for FLOAT fields

Question

I am guessing this has came up before, but I couldn't find the answer to my question. Here is a little code snippet:

    $stmt = $this -> db -> query("
        SELECT
          `Field`
        FROM
          `Table`
        WHERE
          (`ID` = 33608)");
    var_dump($stmt -> fetch());

And this is the result I get:

    array(1) { ["Field"]=> float(1.7999999523163) }

However, the data in the MySQL database is 1.8. The type of the field is float(7,4). $this->db is a PDO object. I have recently migrated to PDO (from AdoDB), and this code was working fine before. I am not sure what went wrong here. Could you point me in the right direction? Thanks!

Answer 1

As documented under Floating-Point Types (Approximate Value) - FLOAT, DOUBLE:

MySQL performs rounding when storing values, so if you insert 999.00009 into a FLOAT(7,4) column, the approximate result is 999.0001.

Because floating-point values are approximate and not stored as exact values, attempts to treat them as exact in comparisons may lead to problems. They are also subject to platform or implementation dependencies. For more information, see Section C.5.5.8, “Problems with Floating-Point Values”

For maximum portability, code requiring storage of approximate numeric data values should use FLOAT or DOUBLE PRECISION with no specification of precision or number of digits.

Therefore, upon inserting 1.8 into your database, MySQL rounded the literal to 001.8000 and encoded the closest approximation to that number in binary32 format: i.e. 0x3FE66666, whose bits signify:

Sign           : 0b0

Biased exponent: 0b01111111
               =   127 (representation includes bias of +127, therefore exp = 0)

Significand    : 0b[1.]11001100110011001100110
                    ^ hidden bit, not stored in binary representation
               =   [1.]7999999523162841796875

This equates to:

 (-1)^0 * 1.7999999523162841796875 * 2^0
=         1.7999999523162841796875

This is the value that is returned by MySQL to the client. It would appear that AdoDB then inspected the column's datatype and rounded the result accordingly, whereas PDO does not.

If you want exact values, you should use a fixed point datatype, such as DECIMAL.